The present invention relates generally to voltage controlled oscillators and, more particularly, to an improved switched element voltage controlled oscillator with an exceptionally wide synthesizer-controlled bandwidth wherein no field tuning is required.
Until recent years, radio transceivers exhibited relatively narrow bandwidths. This was because the frequency determining means was a channel element which inter alia, customarily included a piezoelectric crystal device. Of course several channel elements could be employed to selectively switch to a desired frequency among several. However, the total bandwidth encompassing the plurality of frequencies was not great. In some instances, it was not uncommon that the two frequencies of interest were sufficiently far apart that even though in the same frequency band, i.e. VHF band, for example, a separate and additional radio unit was required to accommodate both frequencies. Such was so even though channel elements were available that could otherwise cover the two frequencies of interest. The filtering required to eliminate harmonic components simply could not be effected.
With the advent of synthesizers, however, channel elements were no longer required, and a significantly wide bandwidth could then be effected. Moreover, any frequency in the entire range was accessible rather than one or two discrete frequencies as determined by the crystal devices. Nevertheless, very wide bandwidths, on the order of 50 megahertz, was not and is not, until now, possible, given the restraints of no substantial degradation in performance specifications, such as, inter alia, sideband noise and modulation capabilities over the designated bandwidth.
It is of course known in the art to employ frequency shift networks in conjunction with voltage control oscillators to shift the operating frequency over a designated range. However, when frequency determining elements are simply switched in, without more tank circuit sensitivity of the oscillator, sideband noise may increase and, in the case of inductive reactance being switched in by, say for example, pin diodes, higher supply line noise modulation sensitivity may occur, as well as possible parasitic oscillation modes.